The present invention relates to a microstrip-line chip antenna element suitable for microwave wireless communications apparatuses such as portable wireless phones and wireless local area network LAN, and an antenna apparatus comprising such a chip antenna element and a communications apparatus comprising such an antenna apparatus.
In microwave wireless communications apparatuses, particularly portable communications apparatuses such as cellular phones, monopole antennas and microstrip-line antennas are generally used for achieving miniaturization and reduction in thickness. A microstrip-line antenna element put into practical use at present has, as described in Japanese Patent Laid-Open No. 10-209740, a radiation electrode formed on an upper surface of a dielectric, rectangular parallelepiped body, high-frequency electric signal being fed from below. FIG. 36 schematically shows the structure of this microstrip-line antenna element. When operated as an antenna, the antenna element is mounted onto a printed circuit board having a ground conductor 96, and a feeding line 94 is disposed on a lower surface of the printed circuit board. An electric line force F is generated between an open end 91 of a radiation electrode 90 and the ground conductor 96, whereby a magnetic flux is generated in a perpendicular direction to the radiation electrode 90, efficiently emitting electromagnetic wave to the space. The length D of the radiation electrode 90 is usually about xc2xc of a wavelength, generating a magnetic flux in a perpendicular direction to the radiation electrode 90 at resonance, the direction of an electric line force F being in perpendicular to the magnetic flux emitted from the end surface 91 of the radiation electrode 90. With respect to the shape of the radiation electrode 90 in a plan view, various shapes such as circle, pentagon, etc. are proposed in addition to rectangle, though vertically or horizontally symmetric shapes are mostly used.
Antennas used for portable communications apparatuses should be small, efficient in radiation and substantially omni-directional. For this purpose, a small antenna element has a structure in which a radiation electrode is disposed on an upper surface or inside of an insulating substrate, because the wavelength of electric current flowing through the radiation electrode is made shorter by influence of the insulating substrate. Because the same radiation effect can be kept even though the radiation electrode is made shorter, the antenna can be miniaturized. The necessary length d of the antenna is represented by the following equation (1):
d=c/(2f0∈r)xe2x80x83xe2x80x83(1),
wherein ∈r is a specific dielectric constant of the insulating substrate, f0 is a resonance frequency, and c is the velocity of light.
As is clear from the equation (1), the length d of an antenna element having a microstrip-line structure can be made shorter as the insulating substrate has a larger specific dielectric constant ∈r at a constant resonance frequency f0. In other words, with a substrate having a high specific dielectric constant ∈r, a small microstrip-line antenna element can be obtained with the same performance. Because a small antenna element is indispensable particularly for cellular phones, etc., the development of smaller, high-performance antenna elements has been desired.
There is an inverted F antenna as an antenna applicable to portable communications apparatuses other than the microstrip-line antenna. The inverted F antenna is constituted by an F-shaped antenna conductor comprising a bent portion at an end connected to a ground conductor plate, a center bent portion connected to a feeding line via a gap. Because the antenna conductor needs only to be as long as about xc2xc of a wavelength, it may be regarded as an antenna having a shape obtained by laterally expanding the microstrip-line antenna element.
The conventional microstrip-line antenna element has the following disadvantages in miniaturization. That is, when the radiation electrode is made smaller by increasing the specific dielectric constant ∈r of an insulating substrate, a resonance bandwidth of the resonance frequency f0 becomes narrower, whereby the antenna is operable only in a narrow frequency range.
This means the restriction of a frequency range available for communications, not preferable for antenna for cellular phones, etc. Accordingly, to develop a practically useful antenna, it should have wide bandwidth characteristics. Particularly in multi-frequency antennas using two or more frequencies, the phenomenon of narrowing a bandwidth is a serious problem, which cannot be controlled only by the properties of the insulating substrate.
A resonance bandwidth BW, a resonance frequency f0 and a Q value representing the performance of an antenna at resonance meet the following relation:
BW=f0/Qxe2x80x83xe2x80x83(2).
The height H a microstrip-line antenna element equal to the thickness of its insulating substrate and the Q value meet the following relation:
Qxe2x88x9d∈r/Hxe2x80x83xe2x80x83(3).
Known as a small microstrip-line antenna is an antenna having a radiation electrode divided to two parts at center, one end of the divided radiation electrode is electrically connected to a ground conductor plate (Hiroyuki Arai, xe2x80x9cNew Antenna Engineering,xe2x80x9d Sogo-Densi Shuppan, pp. 109-112). Because the length of the radiation electrode is about xc2xc of a wavelength at resonance frequency, this antenna is as small as about 50% of the conventional antenna.
Japanese Patent Laid-Open No. 11-251816 discloses a microstrip-line antenna element operable at an expanded bandwidth with a radiation electrode formed on an edge region (adjacent two surfaces) of the substrate. When this microstrip-line antenna element is assembled in a portable communications apparatus, however, a radio wave emitted mainly from the end of the radiation electrode induces electric current in a nearby casing or in conductors on the circuit board, making the current-induced conductors function as an apparent antenna. Thus, the characteristics of this antenna is variable depending on ambient environment, causing impedance mismatching at a feed point and the variation of radiation directivity.
Further, because electronic circuit parts mounted near the antenna element are affected by a high-frequency electromagnetic wave emitted from the end of the radiation electrode, there arise problems of deteriorating communications performance such as noises, errors, irregular oscillation, etc. Conventional means for coping with such problems was to fully separate nearby circuit parts from the antenna element, failing to increase the mounting density of parts near the antenna, thus largely hindering the miniaturization of communications apparatuses.
Accordingly, an object of the present invention is to provide a small microstrip-line antenna element having a sufficient Q value with high gain and broad bandwidth.
Another object of the present invention is to provide an antenna apparatus comprising such an antenna element mounted onto a circuit board with improved mounting density without affecting nearby parts.
A further object of the present invention is to provide a communications apparatus such as a portable information terminal, etc. comprising such an antenna apparatus.
As a result of investigation by simulation to achieve the miniaturization and increase in bandwidth of an antenna element, it has been found: (1) the antenna element can equivalently be provided with a plurality of resonance circuits by properly designing the shapes of a radiation electrode go and grounding electrodes; (2) radiation directivity can be achieved with high gain and without unnecessary field emission by properly designing the arrangement of electrodes; and (3) an area occupied by the antenna can be reduced while providing good antenna characteristics by properly designing the mounting of an antenna onto a ground conductor. The present invention is based on these findings.
Thus, the chip antenna element of the present invention comprises an insulating substrate and a radiation electrode formed on at least one surface of the insulating substrate, the radiation electrode extending from a first end of the substrate or its vicinity to a second end of the substrate or its vicinity, with a width decreasing substantially continuously and/or stepwise.
The chip antenna element according to one embodiment of the present invention comprises (a) a grounding electrode formed on a first end surface and/or a nearby surface region of an insulating substrate, (b) a radiation electrode formed on at least one surface of the substrate, such that the radiation electrode extends from the grounding electrode with or without a gap to a second end of the substrate or its vicinity, with a width decreasing substantially continuously and/or stepwise, thereby having a wide rear end on the side of the first end of the substrate and a narrow tip end on the side of the second end of the substrate, and (c) a feeding electrode formed on at least one surface of the substrate at a position facing an intermediate point of the radiation electrode, with or without contact with the radiation electrode.
The chip antenna element according to another embodiment of the present invention comprises (a) a radiation electrode formed on at least one surface of an insulating substrate, such that the radiation electrode extends from a first end of the substrate or its vicinity to a second end of the substrate or its vicinity, with a width decreasing substantially continuously and/or stepwise, thereby having a wide rear end on the side of the first end of the substrate and a narrow tip end on the side of the second end of the substrate, and (b) a grounding electrode opposing the tip end of the radiation electrode via a gap, and (c) a feeding electrode formed on at least one surface of the substrate at a position facing an intermediate point of the radiation electrode, with or without contact with the radiation electrode.
The chip antenna element according to a further embodiment of the present invention comprises (a) a radiation electrode formed on at least one surface of an insulating substrate, such that the radiation electrode extends from a first end of the substrate or its vicinity to a second end of the substrate or its vicinity, with a width decreasing substantially continuously and/or stepwise, thereby having a wide rear end on the side of the first end of the substrate and a narrow tip end on the side of the second end of the substrate, (b) a first grounding electrode connecting directly or via a gap to the rear end of the radiation electrode, (c) a second grounding electrode opposing the tip end of the radiation electrode via a gap, and (d) a feeding electrode formed on at least one surface of the substrate at a position facing an intermediate point of the radiation electrode, with or without contact with the radiation electrode.
One of the first and second grounding electrodes is preferably in contact with the radiation electrode, whereby the intensity of a radiating electric field decreases in a longitudinal direction of the radiation electrode and increases in a direction perpendicular thereto.
The chip antenna element preferably father comprises an extension electrode connected to the tip end of the radiation electrode and formed on a second end surface of the substrate and/or its nearby region on at least one side surface adjacent thereto. The extension electrode preferably is narrower than the tip end of the radiation electrode.
The insulating substrate is preferably in the form of a rectangular parallelepiped. Also, a ratio W/S of a width W of the wide rear end of the radiation electrode to a width S of the narrow tip end of the radiation electrode is preferably 2 or more, more preferably 2-5. The radiation electrode is preferably formed on adjacent side surfaces of the insulating substrate. Further, the feeding electrode is preferably located at a position deviating from a center of the substrate toward the tip end of the radiation electrode.
The antenna apparatus of the present invention comprises the above chip antenna element mounted onto a circuit board, the radiation electrode of the chip antenna element being in parallel with the edge of a ground conductor of the circuit board, and an open tip end of the radiation electrode being not close to the ground conductor.
There preferably is a gap between the grounding electrode of the chip antenna element and the ground conductor of the circuit board. The feeding electrode is preferably located at a position deviating from a center of the substrate of the chip antenna element toward the tip end of the radiation electrode. The feeding electrode preferably is connected to a feeding line disposed between a pair of ground conductors on the circuit board.
The communications apparatus of the present invention comprises the above antenna apparatus. The communications apparatuses of the present invention may preferably be cellular phones, headphones, personal computers, note-size personal computers, digital cameras, etc. comprising antennas for bluetooth devices.